The present invention is a method of enhancing the perfusion of a porous workpiece with a chemical composition using two or more given fluid solvents under supercritical conditions. The method is particularly well adapted for impregnation and deposition into wood of preservative and other materials which have poor solubility in supercritical carbon dioxide.
The critical temperature of a fluid is the temperature above which liquefaction is not possible at any pressure. Critical pressure is defined as the pressure required to liquefy a gas at the critical temperature. At critical conditions there is no distinction between the liquid and gaseous states. At temperatures and pressures above those at the critical point, fluids are said to be under supercritical conditions. While not truly liquids, they maintain many of the properties of a liquid. However, there are also significant differences. The solvent power of supercritical fluids for various materials is increased significantly above the critical point. When a chemical material is dissolved in a supercritical fluid, the resulting solution appears to have most or all of the characteristics of a true solution. However, the viscosities of these supercritical solutions are very much lower than viscosities of conventional solutions.
The high solvent power of supercritical fluids has found wide commercial application. Among other applications, the technique is used for the extraction of various flavor resins and alkaloids from natural materials. As on example, U.S. Pat. No. 3,806,619 to Zosel discloses the use of supercritical carbon dioxide for extraction of caffeine from coffee. In similar fashion, U.S. Pat. No. 4,104,409 to Vitzhum describes the removal of certain flavoring resins from hops. Among other uses, U.S. Pat. No. 4,354,922 to Derbyshire et al shows the use of a supercritical fluid to extract heavy hydrocarbon oil constituents. The temperature may then be changed or the pressure reduced to precipitate the dissolved hydrocarbon constituents.
Supercritical solutions have also been used for the deposition of various materials onto or into a substrate. Vitzhum, et al, U.S. Pat. No. 4,167,859 teaches extraction of certain aromatic constituents of tea using supercritical carbon dioxide. These constituents are set aside while the tea is moistened and subsequently again extracted with wet supercritical carbon dioxide to remove caffeine. After caffeine removal, the aromatic constituents are then redeposited in the tea from solution in supercritical carbon dioxide.
Berneburg et al, in U.S. Pat. No. 4,552,786, teach the use of supercritical fluids to carry ceramic percursor materials into the pores of a ceramic host in order to fill void spaces and approach the ultimate density of the ceramic material. This is an example which takes advantage of the very low viscosity of supercritical solutions to penetrate what would otherwise be very poorly permeable materials.
West German Offenlegungsschrift 28 53 066 describes the use of supercritical solutions to coat the surface of porous powders or porous articles, such as active nickel catalysts, with an inert protective material. Conditions are regulated so that coatings only a few molecules thick can be applied to the material. In addition to the protection of the active catalysts, the inventors disclose that other porous materials, such as fabrics, can be coated or impregnated with protective or decorative layers. The inventor further notes that by proper selection of extraction and extraction pressure and temperature, the components to be dissolved by the supercritical fluid can be dissolved selectively. Unfortunately, the inventors offer no examples or other data which is specifically informative as to how their process is carried out.
Japanese Kokai SHO 59[1984]-101311 discloses a preservative treatment for wood. This uses either carbon dioxide in the supercritical range or liquid carbon dioxide near the supercritical range as a solvent for the preservative. The inventors claim the advantages of faster permeation of the preservative into the wood, elimination of the need of predrying treatment or incising and, above all, the elimination of the need for waste treatment of residual preservative chemical solutions. The one example in the patent shows the use of a water soluble phenol-group inorganic fluoride wood preservative agent in liquid carbon dioxide to treat beech wood. The composition of the treating material is not further described nor is it made clear whether or not any water was present. In this particular case, the carbon dioxide was approximately 11.degree. C. below the critical temperature at which point it would be a true liquid. Under these conditions the inventors found that the preservative agent had permeated to the center of a cube of wood 10 cm on each side. This would not be surprising even under conventional treating conditions due to the short specimen length and large exposed area of end grain. Liquids are well known to permeate end grain at a rate 10-20 times faster than the rate across the grain. However, if the treatment would work as described for larger specimens, it could potentially be very useful.
Most species of wood are very difficult to impregnate deeply with chemical materials such as preservatives and monomers and polymers. The case of Douglas-fir heartwood can be mentioned here. It has been impossible using normal methods to achieve more than a relatively shallow surface impregnation with usual preservative chemicals. In many cases, after environmental exposure of a treated timber, a thin intact surface shell will remain whereas the center portion has been entirely destroyed by decay organisms or wood boring insects. The same is true for other species of wood which show similar low permeability under the normal vacuum-pressure treatment.
The art does not deal in any helpful detail with the problems encountered when it is desired to impregnate a workpiece with a chemical material not adequately soluble in a single supercritical solvent.